You are here

Moon and Companions

StarDate:

March 20, 2016

A couple of bright lights keep company with the Moon tonight. Regulus, the star that marks the heart of the lion, is close above the Moon as darkness falls. And a much brighter light — the planet Jupiter — stands farther to the lower left of the Moon.

As the night progresses, the Moon will move eastward against the background of more-distant lights. So by the time the Moon sets, shortly before dawn, the distance between the Moon and its two companions will be about equal.

It takes about 27-and-a-half days for the Moon to complete one full turn against the stars and return to the same point against the starry background. That means the Moon covers an angle equal to its own size in a little less than an hour.

From minute to minute, that motion is imperceptible. If you check in every couple of hours, though, you can tell a difference — particularly when the Moon is close to other bright objects, as it is tonight.

And from night to night, the difference can be astounding. When the sky darkens tomorrow night, for example, the Moon will be huddling right next to Jupiter.

The Moon’s motion against the stars isn’t synchronized with its phases, though. As the Moon circles through the stars, Earth and Moon are also circling the Sun. So it takes almost two days longer for the Moon to complete its cycle of phases — a cycle related to the brightest star in the entire sky: the Sun.

Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.